Generally, pigment coated phosphors are widely used as the phosphor material for color cathode ray tubes because pigments having the body colors red, green, and blue can absorb the spectrums departing from the corresponding light wave length band of the phosphor thereby inhibiting the random reflection of the external lights at the screen film and improving the color contrasts.
If excessive amounts of the pigments are coated, however, the emission of the lights from the phosphor is impeded degrading the luminance of the picture. For this reason the amounts of the pigments have to be properly adjusted.
In this respect, the pigment coated phosphor has to have the following characteristics:
1) the adhesion between the pigment particle and the phosphor particle should be strong so that there should be no detachment from the surface of the phosphor during dispersing in the slurry; PA1 2) the pigment particles should be uniformly spread on the surfaces of the phosphor particles, and the flocculation of the pigment particles should be negligible; PA1 3) the pigments should be coated in proper amounts considering the lowering of the luminance and the reflectance of the phosphor particles after the coating of the pigments; and PA1 4) there must be very high compatibility between the pigment coated phosphors and the compounding liquid. PA1 1) Pigment particles coated with arabic rubber or polyvinyl pyrrolidone(PVP) are contacted with phosphor particles coated with gelatin. PA1 2) The polyvinyl pyrrolidone and the gelatin used in process 1 are replaced by a water soluble acidic polymer and a water soluble basic polymer; and PA1 3) An emulsion of acryl series is used. PA1 1) preparing a dispersion solution which is formed by dispersing an acryl emulsion to phosphor particles; PA1 2) preparing a dispersion solution which is formed by dispersing polyvinyl alcohol to pigment particles; PA1 3) uniformly mixing said two dispersion solutions, adding an acid solution to the mixture thereof, and adjusting said mixture to pH 7 to 3; and PA1 4) adding a proper amount of oxalic acid as a curing agent to said mixture, and adhering said pigment particles onto said phosphor particles.
The compounding liquid referred to herein means a liquid to be compounded with pigment coated phosphors to give slurry. The compounding liquid includes a binder such as polyvinyl alcohol, a photosensitizer for the binder such as ammonium dichromate, an organic filter resin, and a liquid vehicle such as water.
As enumerated below, there have been a number of manufacturing processes for the pigment coated phosphor to meet the above described requirements as mentioned in Japanese Laid-Open Patent Publication No. 50-56146, Japanese Patent Publication No. 51-80296 and U.S. Pat. No. 4049845.
In all of the above described conventional pigment coating processes a high viscosity organic bonding agent is used producing undesirable flocculation of the phosphor particles in large quantities.
Thus, if flocculation of the phosphor particles, an insufficiency of the compatibility between the compounding materials, or an insufficiency of the spreading within the compounding liquid occurs, it becomes impossible to obtain good quality phosphor dots or stripes. Further, a worse case can produce absolutely unacceptable phosphor because the adhered particles are not filtered during the sieving process of the compounded phosphor slurry through the use of a mesh.
In order to overcome this problem, multivalent ions are added to phosphors treated with the organic binders. Then the phosphors are subjected to the pH change and temperature change so that the organic binders are cured and the undesirable viscosity is removed. However, the resultant effect is not significant and no practical result has been obtained.